No abstract

-A brief survey of the fabrication, evaluation and uses of organic photoconductive materials as xerographic photoreceptors is given. Included is a discussion of the basic material design roles required for successful operation of photoreceptor devices.The review includes present organic material classes used in commercial. copiers/printers and future potential of organic photoreceptors based on phthalocyanines and squaraines.

A thin organic film of polycrystalline particles of x-metal free phthalocyanine (x-H2Pc) dispersed in a polymer binder, when sandwiched between tin oxide (NESA) and indium electrodes, is shown to exhibit a strong photovoltaic effect. The photovoltaic and rectification properties of In/x-H2Pc/NESA sandwich cells are reported. From the photovoltaic action spectra, the active region responsible for electric power generation was found to be confined to the metal/semiconductor interface. A Schottky barrier width of 300 Å was determined, which allows the capture of 30% of solar irradiance. An electron trap density of 3×1017/cm3 and a Schottky barrier built-in potential of 0.63 V are estimated from C-V measurements. At low voltage, the dark current in the forward direction varies exponentially with voltage: from this dependence values of 2×10−9 A/cm2 and 1.3–2.6 for the saturation current J0 and diode quality factor n are determined. At higher voltage, a super quadratic dependence of forward current on voltage indicated that current conduction is limited by an exponentially decreasing distribution of traps. At peak solar power (135 mW/cm2), a power conversion efficiency (η′) of 1.2% to transmitted light has been obtained. The transmittance of the indium electrode approached 2%. The devices exhibit open circuit voltages Voc of 0.45 V and short-circuit currents of 0.2 mA/cm2 at Air Mass Zero (AMO) sunlight. Therefore, the engineering efficiency of our device approached 0.03%. The monochromatic quantum efficiency of free-carrier generation approached 75% at low light level; this diminished to 30% at solar intensities, characteristic of a cell with large series resistance. The effect of pigment loading, cell thickness, light intensity, binder material, dye sensitization, and the nature of barrier electrode has been studied and optimized.

Thin-film photovoltaic devices consisting of a CdS/trivalent-metal phthalocyanine heterojunction have been prepared. The devices are fabricated by first electrodepositing a thin film of CdS onto a transparent conducting indium-tin-oxide substrate and then depositing phthalocyanine and gold layers sequentially in a vacuum coater. The trivalent-metal phthalocyanines used are chloroaluminium chlorophthalocyanine (ClAlClPc), chloroaluminium phthalocyanine (ClAlPc), and chloroindium phthalocyanine (ClInPc). Under an AM2 illumination of 75 mW cm−2, these heterojunction devices produce an open-circuit voltage Voc of 0.70 V and short-circuit current Jsc of 0.8 mA cm−2. The conversion efficiency is about 0.2%, which represents one of the highest values reported for phthalocyanine photovoltaic devices at high light intensity.

We have measured the direct current (dc) action spectra of A1 and In/phthalocyanine ( a , P, and s-H?Pc and VOPc) photovoltaic cells in the wavelength range 400-900 nm. The data were analyzed in terms of both the local carrier generation and excitqn diffusion models. According to the local carrier generation model, photoactive barrier widths w of 390, 475, 310, and 950 A for In/u, P, s-HzPc, and VOPc, respectively, were obtained. Thc depletion region width of In-phthalocyanine Schottky junctions were independently measured using a dc discharge capacitance technique. In all cases. the depletion region widths (capacitance) were consistently smaller than the photoactive collection region widths (action spectra).~hese results suggest that excitons produced within a diffusion length of the barrier can diffuse into the barrier zone and dissociate into collectable~carriers. Using a combined local carrier gener:tion and exciton diffusion model, a barrier width of the order of 200-300 A and an exciton diffusion length of 300-500 A was estimated for In/x-HzPc cells.